Femtosecond-laser-driven millimeter-wave signals as probes for high-field transport dynamics in semiconductors.

摘要

This dissertation combines the study of carrier transport dynamics in semiconductors with an investigation of the propagation characteristics of millimeter-wave transmission lines, utilizing both free-space and guided terahertz electromagnetic pulses. For the generation and measurement of these picosecond-duration pulses, femtosecond lasers have been employed, since purely electrical methods are not readily available to produce or detect a signal varying on a time scale of a picosecond.;Various lasers have been employed for these kinds of experiments, and they fundamentally limit the achievable signal-to-noise ratio and time-resolution of the measurements. Therefore, initially, the noise characteristics of a self-mode-locked Ti:sapphire laser and a colliding-pulse mode-locked (CPM) laser have been studied in detail and compared.;Free-space and guided electrical pulses have first been used in the study of the transient velocity overshoot phenomenon in GaAs and Si. The time-domain waveforms of radiated and guided pulses are proportional to the acceleration and velocity of carriers in the semiconductor materials, respectively. These measurements have been analyzed quantitatively to give the electric-field and initial-carrier-energy dependences of velocity overshoot in GaAs for electric fields up to 200 kV/cm. The terahertz radiation technique has also been applied to study Si, and, for the first time, the transient velocity overshoot in Si has been directly observed experimentally.;Coplanar guiding structures also need to be characterized both for deembedding the transmission-line response in the velocity overshoot study and understanding the signal propagation behavior in solid-state circuits. The substrates for the circuits are often lossy and dispersive for the propagation of high-frequency signals. Therefore, we have studied the dynamics of millimeter-wave signal propagation on coplanar striplines fabricated on lossy semiconductor substrates. A model incorporating the effect of a conductive substrate through the loss tangent has been developed and verified by a picosecond pulse propagation experiment using the electro-optic sampling technique for the measurements.;The time resolution of the measurement techniques used is ultimately limited by external factors such as carrier lifetime and electro-optic resonance. Therefore, a new technique for the measurement of picosecond electrical signals, with a time resolution limited only by the laser pulse width, has been devised using a photoconductive step-function gate.